add http proxy support

## Agent Command Line

```
HTTP_PROXY=http://127.0.0.1:8080 \
  rackspace-monitoring-agent
```

## Apache Commandline

```
httpd -f $PWD/httpd.conf -DFOREGROUND -e info
```

## Apache Config
```
LoadModule authn_file_module libexec/apache2/mod_authn_file.so
LoadModule authn_dbm_module libexec/apache2/mod_authn_dbm.so
LoadModule authn_anon_module libexec/apache2/mod_authn_anon.so
LoadModule authn_dbd_module libexec/apache2/mod_authn_dbd.so
LoadModule authn_default_module libexec/apache2/mod_authn_default.so
LoadModule authz_host_module libexec/apache2/mod_authz_host.so
LoadModule authz_groupfile_module libexec/apache2/mod_authz_groupfile.so
LoadModule authz_user_module libexec/apache2/mod_authz_user.so
LoadModule authz_dbm_module libexec/apache2/mod_authz_dbm.so
LoadModule authz_owner_module libexec/apache2/mod_authz_owner.so
LoadModule authz_default_module libexec/apache2/mod_authz_default.so
LoadModule auth_basic_module libexec/apache2/mod_auth_basic.so
LoadModule auth_digest_module libexec/apache2/mod_auth_digest.so
LoadModule proxy_module libexec/apache2/mod_proxy.so
LoadModule proxy_connect_module libexec/apache2/mod_proxy_connect.so
LoadModule log_config_module libexec/apache2/mod_log_config.so
LoadModule log_forensic_module libexec/apache2/mod_log_forensic.so
LoadModule logio_module libexec/apache2/mod_logio.so

Listen 8080
NameVirtualHost *:8080
User ryan.phillips

ProxyRequests On
AllowCONNECT 443 80
ProxyVia On

<Directory proxy:*>
  Order deny,allow
  Deny from all
  Allow from 127.0.0.1
</Directory>

LogLevel debug
ErrorLog "|cat"
LogFormat "%h %l %u %t \"%r\" %>s %b" common
CustomLog "|cat" common
```