Some bugfixes and enhancements

Hi, I did some bugfixes and enhancements for this projects, include:

* bugfixes:
  * compare COMMAND string case insensitively: some client sending "GET" while some sending "get".
  * get_int bugfix
  * check if the command is read compleled or not before do it
  * init data_buffered to 0 when create a connection in `new_connection`
  * bad realloc usage
* enhancements:
  * run as daemon
  * command-line options, see redis-leveldb -h
  * incrby command
  * use gmp to deal bignum in incr/incrby
  * largen the read buffer

All above are in this Pull-Request, and after this, I rewrite this project (in c++) and make it easier to implement new redis-command in the master branch of my fork (https://github.com/appwilldev/redis-leveldb/tree/master), now it supports these redis commands:

* incr/incrby
* get/set
* mget/mset
* multi/exec/discard
* keys

If you accept, I'll send a new Pull-Request of the rewriting. 
Thanks for your work.