Merge pull request #238 from sdaubert/master

Add documentation to Struct ans its descendants
Properly indent code in README.md and add to requirements
Fix #239 by raising an error if the library list supplied to ffi_lib is empty
Add rbffi_gc_mark_locations() that behaves like rb_gc_mark_locations(), for VMs that do not have it.
Correct some rb_global_variable() calls that were passing in stack addresses.
Struct classes store @layout in a class ivar, not a cvar.
Enable loading FFI gem on rubinius
Bump version to 1.3.0.dev
Fix #242 by coercing :string parameters using #to_str
Ignore the libffi directory for RDoc

This directory does not contain useful documentation for rubyists so
RDoc should be told to ignore it.

See also rdoc/rdoc#165
Merge pull request #243 from drbrain/master

Ignore ext/ffi_c/libffi for RDoc
Use Rubinius API on Rubinius for forcing GC
Merge pull request #244 from dbussink/master

Use Rubinius GC api for forcing GC
Add to_str and to_ptr benchmarks
Remove rbffi_gc_mark_locations()
Bump version to 1.3.0
Possible fix for #245 - passing nil args into :string params did not advance argidx.
Bump version to 1.3.1
Add tests for block-form of Buffer::new and MemoryPointer::new
Add Travis CI build status image

[ci skip]