Fixing varint and decimal encoding/decoding

[ahisbrook]

Found several numbers which were encoding/decoding incorrectly. The issue was in append_varint.

Example:
append_varint(1042342234234); read_varint returns -57169393542

Also made a small change to read_decimal which would occasionally fail to decode decimals between -1 and 0.

Example:
append_decimal(-0.031108612692221094); read_decimal returns 0.0

[iconara]

Thanks for hunting down these bugs, I'll merge this in the morning and push up a new gem.

[iconara]

Merged through 31d5520. v2.0.3 is up on RubyGems.