support additional experimental typechecker options

fix test path in travis.sh

Support file-level constants

Add HackBuilderValues::code()

fixes hhvm#69

fix support for Imm(Map|Set|Vector)

update docs

Add examples/ to export-ignore

Add sprintf versions of script-related functions

Add HackBuilderKeys::lambda() and HackBuilderValues::lambda()

More convenient than subclassing for one-offs.

fixes hhvm#31

Update type-assert dependency to a version that supports 3.18