Allow custom buffer decoders, fix issue #12 

Addresses this issue: https://github.com/wvanbergen/node-vertica/issues/12

connections options can have a decoders hash.
such as
  decoders =
    boolean : (buffer) -> "this guy decodes boolean field buffer"
    integer : (buffer) -> "this guy decodes integer field buffer"

Query passes the decoders to Query.Field, and Query.Field tries to find an appropriate decoder from the custom decoders first, otherwise falls back to the driver's own set of decoders.

This can be used easily to fix decoding behavior for 64bit or infinite precision numeric. The user will just have to substitute node-num or any other decoder to handle buffer deserialization.