Safer implementation of Swap()

Also added documentation, unit tests for swapping Document/Value,
Document/Document.

Miscellaneous cleanups

The error messages in ParseNumber used `is.Tell` to report the
position of the number parsing error.  Depending on the copy
optimization of the current stream, this can lead to different
behaviour (beginning of number vs. position of error).

 * Delegate constant string construction to SetStringRaw
 * Delegate "const Ch*" overloads to GenericValue variants
   of operator[], FindMember and RemoveMember
 * Remove repeated template arguments in nested struct Array

(cherry-picked from ca9b0332d)

Instead of hard-coding the value 0 for the parseFlags in the
various parsing overloads, explicitly use kParseDefaultFlags
to provide more self-documenting code.

@pah

Added GenericValue::Swap()

Add explicit MemberIterator, return MemberEnd() from FindMember()

No functional changes.

 * use MemberIterator instead of plain pointer
 * check against MemberEnd() instead of NULL

In order to activate the suppression of "-Weffc++" warnings in the
template meta function classes (non-virtual destructor), move the
inclusion of the meta-function header `internal/meta.h` after the
suppression pragma.

Add dedicated class-based member iterator to prepare the switch to a
(safe) API change to return MemberEnd() from FindMember().

Pointer-based iterator can be kept by defining
RAPIDJSON_NOMEMBERITERATORCLASS. This may be useful for platforms without
a working <iterator> header.

 * Add/RemoveConst
 * IsSame, IsConst, IsMoreConst
 * Enable/DisableIf (including helper macro)

GenericReader: simplify local stream copy optimization

Eventually move to internal header `internal/streamcopy.h`?

Based on 088e8173.

rapidjson.h:
 * StreamLocalCopy: add default argument to copy optimization selector
   based on StreamTraits of Stream parameter
 * drop operator->, operator*
 * make Stream (reference) member public
 * drop empty destructor

reader.h:
 * add local references, initialized from "copy"
   (reverts algorithmic bodies back to plain 's.xx()')

This reverts commit 63757f64.

Fix/Suppress all -Wall -Wextra -Weffc++ warnings on gcc 4.6.x - 4.8.x

Fixes local copy optimization

The previous optimization #32 has problem that restoration requires
assignment operator.
Change the backup/restore process using a template wrapper class to
select code path.

Adds Valgrind memcheck process to Tavis CI