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

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

For more details see: https://github.com/miloyip/rapidjson/issues/132

This commit tries to minimize the required code changes and forwards the `Handler::Key()` calls to `Handler::String()` wherever possible in order to not break existing code; or at least not code deriving from `BaseReaderHandler` when implementing a custom `Handler`.

Initial C++11 move support

GenericValue: accept values with different allocators for read-only access

This shall generate best possible precision (if strtod() is correctly
implemented). Need more unit tests and performance tests. May add an
option for accepting precision error. Otherwise LUT in Pow10() can be
reduced.

GenericValue: reduce growth factor for array/object reallocations

short string optimization

The `ShortString` can represent zero-terminated strings up to `MaxSize` chars (excluding the terminating zero) and store a value to determine the length of the contained string in the last character `str[LenPos]` by storing `MaxSize - length` there. If the string to store has the maximal length of `MaxSize` (excluding the terminating zero) then `str[LenPos]` will store `0` and therefore act as the string terminator as well. For getting the string length back from that value just use `MaxSize - str[LenPos]`.

This allows to store `11`-chars strings in 32-bit mode and `15`-chars strings in 64-bit mode inline (for `UTF8`-encoded strings).

Suggested-by: @miloyip

Instead of replicating the functionality of `GetString()` and `GetStringLength()` in `StringEqual()` it now calls these methods instead.