Refactor XML parser using prototype pattern
Change as_present() and as_absent() to omit(bool)
Added parsing "define" XML element for FAST 1.2
generate omit_xxx() for optional fields
Remove code to support arena_allocator for fields. 

My experiment showed that using arena_allocator as the field allocator did not improve decoding performance. Remove the support to avoid code complexity.
Added composite_type_test
Fixed some problems to compile in C++11 mode.
Added enum support
Fixed several parsing problems and add boolean type.
Update ReadMe
Fixed problem where boolean type not exported in dynamic library build
Support adding export macro to generated classes.
Fixed problem for parsing enum field operator
Fix problem where the accessors for enum are not generated correctly.
Fixed problem to parse decimal field with exponent element only.
Fixed generating sequence with only one field.
Another fix to generate decimal field with const exponent.
Fixed problem for generating initial value for byteVector
Rename composite_field to composite_type
Add clone() member function to field_instruction