Solving jira problem (IOTDB-298) Refactor the "last" and "first" aggregators

[thss15_yit]

Hi,
    I have been working on JIRA issue [IOTDB-298], and now in the following 
pull request I change the aggregation function's name from first,last to 
first_value,last_value to make it more clear, and change the relevant 
documents. 
    The pull request is https://github.com/apache/incubator-iotdb/pull/594, 
    Thanks for your checking.
    I'm going to complete a new 'last' function to get the last point(s) on 
this issue. The function still needs to be discussed. Thanks for your idea.
Tao Yi