Add ability to set number of parallel runs (closes #27) (#28)

Author: Lukas-Kullmann

.gitignore

@@ -2,3 +2,4 @@ lib
 node_modules
 .screenshots
 .idea
+*.iml

README.md

@@ -14,6 +14,9 @@ Before using this plugin, save the BrowserStack username and access key to envir
 
 Project name and build name will be displayed in BrowserStack if you set the environment variables `BROWSERSTACK_PROJECT_NAME` and `BROWSERSTACK_BUILD_ID`.
 
+If you have troubles starting multiple browsers at once, or get `browserstack-local` related errors like [#27](https://github.com/DevExpress/testcafe-browser-provider-browserstack/issues/27),
+try setting the `BROWSERSTACK_PARALLEL_RUNS` environment variable to the number of browsers you want to run simultaneously, or to 1 if you want to run just one browser. 
+
 You can determine the available browser aliases by running
 ```
 testcafe -b browserstack

src/index.js

@@ -75,15 +75,18 @@ function getProxyOptions (proxyConfig) {
 
 function createBrowserStackConnector (accessKey) {
     return new Promise((resolve, reject) => {
-        var connector = new BrowserstackConnector();
-
+        var connector    = new BrowserstackConnector();
+        var parallelRuns = process.env['BROWSERSTACK_PARALLEL_RUNS'];
+        
         var opts = {
             key:             accessKey,
             logfile:         OS.win ? 'NUL' : '/dev/null',
             forceLocal:      !!process.env['BROWSERSTACK_FORCE_LOCAL'],
             forceProxy:      !!process.env['BROWSERSTACK_FORCE_PROXY'],
             localIdentifier: Date.now(),
 
+            ...parallelRuns ? { parallelRuns } : {},
+
             //NOTE: additional args use different format
             'enable-logging-for-api': true
         };