Update install task in makefile
Seperate api responses into separate file
Make PingdomError public

Also rename some private types
Add tests for PingdomError
Check list/read methods return CheckResponse instead of Check

Since the returned value from the pingdom API is not always a
complete check it should be more clear to return an explictly
different type of object.

This will also make it easier to support polymorphic Check types
as input to Create/Update since the Read/List responses won't differ.
Turn Check into interface to allow for future check types

Old Check struct is now HttpCheck
CheckService create/update methods take Check interface instead of HttpCheck
Remove read-only fields from HttpCheck type
Add PingCheck type

Also update documentation with new usage examples
Move specific check types into separate file