Import some general-purpose changes from Metron into FastClick

- General documentation fixes
- Avalaibility of JSON library is exported
- HashSwitch is batch-compatible
- PaintSwitch is batch-compatible
- String have trim_space_left and replace
- Add statistics handlers for DPDK elements

Changes are mostly from Georgios Katsikas, RISE SICS and Tom Barbette (ULiege)
10 jobs from master in 9 minutes and 57 seconds
Status Job ID Name Coverage
  Test
passed #10003
autobatch

00:02:06

passed #10002
batch

00:02:01

passed #10006
dpdk-batch

00:01:48

passed #10007
dpdk-nobatch

00:01:48

passed #10004
ip6

00:01:57

passed #10005
mt

00:02:10

passed #10009
netmap

00:01:47

passed #10010
netmap_nopool

00:02:14

passed #10011
netmap_nopool_nozc

00:02:16

passed #10008
netmap_single

00:01:36