move LWP::UserAgent->new() into a function
various simplification and code duplication removal changes

o XXXtoYYY functions gutted and replaced with BtceConversion('XXX_YYY') which
  does the same as canned functions but with more flexibility (a user can now
  specify any currency exchange that btc-e.com supports, not just the three
  hardcoded functions). I preserved the hardcoded functions for backwards
  compatibility, but it is likely they will be deprecated in the future.

o eval and catch issues decoding json info, the wire is not infallable!

o move creation of a new agent inside a function for abstraction purposes;
  technically not needed now that _apiprice is now the one place it is called
  (for now), I anticipate further places to call it
Merge pull request #1 from toddfries/master

enhance module by providing user specified exchanges, do some refactoring, code deduplication