GenericValue: improve documentation for operator[]

GenericValue: improve operator[] disambiguation

In the original disambiguation fix for `GenericValue::operator[]` (#170),
the documentation has been missing, which led to quite badly rendered
Doxygen pages.

During a cleanup, I've realized that a much simpler disambiguation is
possible:

````cpp
  GenericValue& operator[](SizeType idx); // array
  template <typename T>
  GenericValue& operator[](T* name);      // object
````

This approach works, as non-template functions are preferred over
template functions.

In order to improve the error messages, the pointer type is restricted
to `(const) Ch`.

Update `tutorial.md` to drop the ambiguity warning.

Demonstrate CRTP in SAX example

Disambiguate GenericValue's [0] and ["string"]

Closes: https://github.com/miloyip/rapidjson/issues/142

.travis.yml: avoid Valgrind bug with '-march=native'

Fix MemoryStream::Peek() and add test for fix

Use -march=native for platform-specific optimizations

Add perf tests for EncodedInputStream and AutoUTFInputStream

Use predefined macros to enable SSE with gmake

MemoryStream::Peek() did not return '\0' if src_ == end_, but Peek() == '\0' is
used in parsing in the GenericReader. Without this change, parsing with
MemoryStream as the InputStream could result in a segmentation fault.

Correct `PushBack()` example

Fix small typo in the docs

turn implicit integer conversions into static_casts to avoid warnings

Alternative compilation fix for Solaris

Some compilers do not export the standard C library functions
to the global namespace, in case the C++ header variants are
included (<cstdlib>, <cstring>).

RapidJSON currently uses:
 * malloc, realloc, free
 * memcpy, memmove, memset, memcpy

Add an explicit namespace qualification to avoid lookup problems.

make MemoryPoolAllocator's Malloc work after Clear if a buffer was not supplied by the user

Stack: adjust growth factor

The growth factor for the `internal::Stack` helper has not been updated
together with the growth factors used in GenericValue (#130).

Discovered in https://github.com/miloyip/dtoa-benchmark/issues/1 with
assertion fail.

fix `SAX` documentation to also mention the new `Key()` method in the `Handler` concept

add `Key()` method to the `Handler` concept

It was a copy-n-paste error for the last argument of `Key()` and `String()`...