Added new http_header module

See PR #3163

This module shows and stores http headers returned by hosts. You can specify some headers to exclude from reporting.

Sample output:
```
firefart@LinuxMint ~/Coding/metasploit-framework $ ./msfcli auxiliary/scanner/http/http_header RHOSTS=www.firefart.net E
[*] Initializing modules...
RHOSTS => www.firefart.net
[*] Scanned 1 of 2 hosts (050% complete)
[*] Scanned 2 of 2 hosts (100% complete)
[*] Auxiliary module execution completed

firefart@LinuxMint ~/Coding/metasploit-framework $ ./msfcli auxiliary/scanner/http/http_header RHOSTS=www.firefart.net VERBOSE=true E
[*] Initializing modules...
RHOSTS => www.firefart.net
VERBOSE => true
[*] Requesting 87.230.19.86:80
[*] 87.230.19.86:80: Deleted Header Vary
[*] 87.230.19.86:80: Deleted Header Date
[*] 87.230.19.86:80: Deleted Header Content-Length
[*] 87.230.19.86:80: Deleted Header Etag
[*] 87.230.19.86:80: Deleted Header Accept-Ranges
[*] 87.230.19.86:80: CONTENT-TYPE: text/html
[*] 87.230.19.86:80: LAST-MODIFIED: Sun, 29 Dec 2013 16:05:40 GMT
[*] 87.230.19.86:80: SERVER: Apache
[*] Scanned 1 of 2 hosts (050% complete)
[*] Requesting 2a01:488:66:1000:57e6:1356:0:1:80
[*] Scanned 2 of 2 hosts (100% complete)
[*] Auxiliary module execution completed
```

Steps to verify:
- [ ] Checkout
- [ ] Scan HTTP Server
- [ ] Verify with manual `curl -I` or smth. similar