annPeak_bytss: annPeak_bytss
In xons/chipMap: Make heatmap of High throughput sequencing data

Annotate peak by it's nearest transcription start site. If peak overlaps multiple tss, random one will be selected.

1	annPeak_bytss(peak.gr, tx, tssup = 1000, tssdown = 1000)

`peak.gr`	A peak GRanges. This can be get from Bed file of peak using bed2gr function in this package.
`tx`	A data frame of transcript/gene annotation, contains columns 'txid','seqnames', 'start', 'end', 'strand'.
`tssup`	Window size around TSS that will be defined as overlap_tss, and 0 distance will be given. Default is 1000, means must overlap within 1 kb upstream of TSS.
`tssdown`	Window size around TSS that will be defined as overlap_tss, and 0 distance will be given. Default is 1000, means must overlap within 1 kb downstream of TSS.