add proxy capability

This adds proxy'ing capability.
Updated README.
Also, removes one unused Net::HTTP::Get.new line.

## Tested scenarios
1. Bad proxy host or port (running localhost:8080):
```1.9.3-p547 :003 > pd = PagerDuty::Full.new(apikey = 'MY_KEY', subdomain = 'MY_DOMAIN', http_proxy = "http://localhost:8081")```
``` => #<PagerDuty::Full:0x007fa5739b79c0 @apikey="MY_KEY", @subdomain="MY_DOMAIN", @http_proxy=["http://localhost:8081"]>```
```1.9.3-p547 :004 > sched = pd.Schedule.search().length```
```Errno::ECONNREFUSED: Connection refused - connect(2)```

2. Good proxy host and port:
```1.9.3-p547 :005 > pd = PagerDuty::Full.new(apikey = 'MY_KEY', subdomain = 'MY_DOMAIN', http_proxy = "http://localhost:8080")```
``` => #<PagerDuty::Full:0x007fa573960508 @apikey="MY_KEY", @subdomain="MY_DOMAIN", @http_proxy=["http://localhost:8080"]> ```
```1.9.3-p547 :006 > sched = pd.Schedule.search().length```
``` => 4```

3. No proxy:
```1.9.3-p547 :007 > pd = PagerDuty::Full.new(apikey = 'MY_KEY', subdomain = 'MY_DOMAIN')```
``` => #<PagerDuty::Full:0x007fa57483f3a8 @apikey="MY_KEY", @subdomain="MY_DOMAIN", @http_proxy=[]>```
```1.9.3-p547 :008 > sched = pd.Schedule.search().length```
``` => 4```