- Improved performace

- Refactored
- Fixed verify checksum
- Made evm more stable stable

Release v0.1.8

Release v0.1.7, merge PR from @CalvinWalzel to fix the stop command

Support installing Elasticsearch pre-release versions

v0.1.5_3: fix stop issue #4

Fix create PID file while starting ES

Check if downloaded file exits before installing

1) Verify checksum of downloaded file.

2) Added commands:
  - status: Print running status of current Elasticsearch verion
  - stop: Stop Elasticsearch if running
3) `start` command now starts Elasticsearch in the background
4) Replace `wget` with `curl`
5) Bugs fixed & refactoring

1) Add `evm which <version> command`

2) Parse options with getops function
3) Validate arguments
4) Bugs fixed & improvement