Rmetl

1. Fix some minor bugs;
2. Improve description of rMETL usage;

In next version, I will add other aligner and more TE concensus for detection of MEI.

rMETL update:
1. Readjust the framework of rMETL to save memory usage.
2. Properly add code annotations to make it more readable.

Any suggestions/reports are welcome.

Thank you

Jiang Tao