deleted redundant unsafe_load_file (fixes #55)
check for DateTime#to_time w/o instantiating
updated Parse::Date to account for DST

wait for it...
...actually, the previous commit was a mistake.

I realized it was a mistake before even committing it, but
I'm weird and like to keep a history of what I've done even
if it was wrong.

It turns out DST is accounted for. The problem was actually
the test, which expected the gmt_offset to always be the
CURRENT local GMT offset. In fact it should be the GMT
offset for the given date (which it is).

This reverts commit 6a6a90c04d5227ffcbda8863a88da229b7e1e4ac.
fixed Transform::ToDate spec w/ the right expectation re: DST
added check for libyaml version when using Psych (see #56)
made PREDEFINED_TAGS a const instead of an instance variable