Added const_iterator to StringData. Supports all operations that a const char* can support

Signed-off-by: Andrew Morrow <acm@10gen.com>

Conflicts:
	src/mongo/base/string_data.h
SERVER-12157: fix build on Linux 32-bit and old version(s) of g++.

Conflicts:
	src/mongo/base/string_data_test.cpp